1. Field of the Invention
The present invention relates to an optical scanning apparatus, and more particularly, to an optical scanning apparatus capable of preventing image print quality from being degraded due to a change in temperature therewithin.
2. Description of the Related Art
Generally, an optical scanning apparatus, such as a laser scanning unit (LSU), is applied to an image recording apparatus, such as a copy machine, printer, or facsimile, which prints an image on printing paper. The optical scanning apparatus scans a photoreceptor medium of an image forming apparatus with a light beam emitted from a light source, such as a laser diode, to form an electrostatic latent image.
FIG. 1 is a diagram of a conventional optical scanning apparatus. Referring to FIG. 1, the conventional optical scanning apparatus includes a semiconductor laser 2, a collimating lens 3 to convert a laser beam diverging from the semiconductor laser 2 into a converging laser beam, a cylindrical lens 5 having a predetermined refractivity only in a sub scan direction, a light deflector 7 to deflect a light beam, an f-θ lens 9 disposed between the light deflector 7 and a target surface 1, i.e., a photoreceptor drum. Reference numeral 4 denotes an aperture iris to adjust the diameter of the laser beam passing therethrough.
Since the cylindrical lens 5 has the predetermined refractivity only in the sub scan direction, it makes a laser beam passing through the aperture iris 4 eventually form a linear image on a deflection surface 7a of the light deflector 7 in a main scan direction.
The light deflector 7 is rotated by a driving unit, such as a motor, at a predetermined speed during an image formation operation. The light deflector 7 may be a polygon mirror having a plurality of reflective surfaces used as the deflection surface 7a. 
The f-θ lens 9 corresponds to an image forming optical system and is displaced from the center between the light deflector 7 and the target surface 1 to be closer to the light deflector 7. The f-θ lens 9 has different refractivities in the main and sub scan directions and guides a laser beam deflected and reflected by the deflection surface 7a of the light deflector 7 to the target surface 1.
In a conventional optical scanning apparatus having the above-described structure, the light deflector 7 is rotated so that the target surface 1 is scanned with a light beam in a main scan direction, thereby recording an image on the target surface 1.
With the recent high performance, small size, and low cost of image recording apparatuses, such as laser printers, laser facsimiles, and digital copy machines, optical components have been made of a plastic material. It is difficult to simultaneously satisfy these recent trends and realize satisfactory image print quality using the conventional optical scanning apparatus. In a case where the cylindrical lens 5 is made of a plastic material, when the internal temperature of the optical scanning apparatus increases due to the operation of a driving unit within the optical scanning apparatus and the operation of the image recording apparatus, the refractive index or shape of the cylindrical lens 5 changes due to the expansion of the material of the cylindrical lens 5. Consequently, the diameter of a beam on the photoreceptor medium increases, or the position of the beam on the photoreceptor medium changes, so that image print quality is degraded.
It is possible to manufacture the cylindrical lens 5 using a material having a small change in the refractive index and a small coefficient of expansion. However, this material is expensive, thereby increasing the manufacturing costs, so it is difficult to satisfy the requirement of low cost.
Furthermore, in the conventional optical scanning apparatus, an optical path between the semiconductor laser 2 and the light deflector 7 is on a straight line. Accordingly, the freedom in designing an apparatus and disposing components is restricted.
Moreover, in the conventional optical scanning apparatus, a laser beam emitted from the semiconductor laser 2 must be incident on the deflection surface 7a of the light deflector 7 at a large angle in order to secure a space for assembling or adjusting components. Thus, many factors may degrade image print quality. For example, there may be a difference in the quantity of light between the center and ends of an image due to differences in reflectance of the deflection surface 7a, or an effective area of the deflection surface 7a may increase.